Why use diodes to add a constant voltage drop on the output of a constant current source, when a resistor would work just as well?

Yes, I had intended to mention that, but forgot, oh well. I had mentioned a diode in a previous video, and I wanted to show that you wouldn't get the expected voltage drop at the lower current.It's probably stupid to show that, but it is what it is. Oh, and I was going to add some annotation too to mention that, but of course forgot to do that as well because I'm double stupid.The final circuit I built already has a resistor instead.

Heya Dave, I for one would like to learn about noise reduction in test setups. I've seen some people in a physics lab using grounded aluminum foil around their test leads to reduce noise. I think it would be interesting to see how the pros do it.

Why use diodes to add a constant voltage drop on the output of a constant current source, when a resistor would work just as well?

Yes, I had intended to mention that, but forgot, oh well. I had mentioned a diode in a previous video, and I wanted to show that you wouldn't get the expected voltage drop at the lower current.It's probably stupid to show that, but it is what it is. Oh, and I was going to add some annotation too to mention that, but of course forgot to do that as well because I'm double stupid.The final circuit I built already has a resistor instead.

It was interesting to see the diode in operation and find out why it worked and under what circumstances it appeared not to work. Time well spent imo.

Any benefits/problems fixing the rail to rail issue by running the op amp from a separate supply? Say, 15V (+/-?) for the ref and +/-18V for the opamp.

Well, the VHP100 are supposed to be superior over the HZ series ones, as confirmed by a Vishay representative.

I bought 5EA of the Z foil types, i.e VHP202Z, to use them as standards.But initially they were a bit of disappointing for me, having relatively high T.C. between 0.3 and 1.0ppm/K.

They have 9.9998k, 0.001%, 80€ each.Adjusted to exactly 10k by comparison against an SR104, in the end.As each resistor assembly is equipped with a thermometer, I was able to determine each individual T.C. = dR/dT, which mitigates the unceratainty due to temperature.

Their ageing drift against each other is well below 1ppm over 4 years, until now.Superb.

Not stupid, you've simply contracted CRS, which is among things that happen when you grow old (which is a damn-fool thing to do.) So, from now on, any day that you don't let the magic smoke out is a good day, and you did set a current limit on your PS.

Anyway... What's the noise spec for the 10M version of the resistors? Prolly not an issue with 1uA and 0.05% though, but...

That's the kind of video I would love to see all the time at EEVBlog.I hope you spend more time this season recording/editing/publishing them.Discussing about the secrets of a good design is more instructive than any theory masterclass.

Sorry to dredge up this old thread, but I'm trying to breadboard this constant current source, and need to confirm where the series of 1, 2 or even 3 diodes go. Are they in series with the load resistor and ground? And is the V- pin on the opamp also to go to the anode of the top-most diode? The attached drawing shows how I'm thinking is the correct circuit.

Sorry to dredge up this old thread, but I'm trying to breadboard this constant current source, and need to confirm where the series of 1, 2 or even 3 diodes go. Are they in series with the load resistor and ground? And is the V- pin on the opamp also to go to the anode of the top-most diode? The attached drawing shows how I'm thinking is the correct circuit.

Thanks,bg

It doesn't seem to be correct to lift the negative opamp rail with the diode voltage drop. The idea of diode is to lift the positive opamp input up a bit so it doesn't operate with voltages close to the power rails.

So from my understanding you need to use diodes in series with only the load, leaving the opamp rails as is.

It has been discussed in the beginning of this thread: you can use one resistor in stead of 3 diodes.Because the current going through the load is constant, you can calculate what resistor you'd need to create the desired voltage drop using Ohm's law.

One quick question. I tried to make one of these the other day, but since I didn't have the REF102 chip I used a REF195G instead (5v reference) and adjusted the resistor for the current I needed.

I know the precision won't be the same, but the thing is that it doesn't even work. When I measure the voltage reference between brief and "GND" of the REF195G I dont get the 5V I am supposed to get.Is there any special thing about the REF102 that other voltage references like the REF195G dont have that makes this circuit only work with REF102?

I am using this precision current source to generate 10 uA current. Op amp is OP1177, REF is REF102. The current changes by 0.5% when the load resistance changing from 50 kOhm to 150 kOhm, but I use unscreened wires. The output current contains ac contribution that depends on the load resistance (approximately 0.1 V). How to get rig of this AC contribution? Is it a tycal accuracy for this current source? How can I additionaly improve the accuracy of current?

I am using this precision current source to generate 10 uA current. Op amp is OP1177, REF is REF102. The current changes by 0.5% when the load resistance changing from 50 kOhm to 150 kOhm, but I use unscreened wires.

I measured the current using the ammeter Keithley 6517B connected in serial with load. Also I measure the current throught a voltage drop on the precision 100k resistor connected in serial with load. The current drops with the increase of load resistance.